Brachypodium distachyon as a model to study nonhost resistance to wheat stripe rust
The model grass Brachypodium distachyon has been used to study nonhost resistance mechanisms to the wheat stripe rust pathogen, Puccinia striiformis f. sp tritici. Numerous B. distachyon accessions were screened with an array of UK and Australian P. striiformis isolates and distinct infection phenotypes identified, ranging from complete resistance to partial susceptibility. Three mapping families were established - BdTR10H x TEK4, BdTR13K x Bd21 and ABR6 x Bd21 - and immunity was dominantly inherited when they were tested with one Australian and three UK isolates. Depending upon the mapping family, between one and three genes for stripe rust resistance were present and designated Yrr1 to Yrr3. Yrr1, which is present in all three families, was effective against all isolates and was fine mapped to a 100 kilobase region containing six candidate genes. Interestingly, no candidate was homologous to a known resistance gene. Yrr2, which is present in the BdTR13K x Bd21 and ABR6 x Bd21 families, is race-specific and was mapped to a 1 megabase region that contains multiple, classic NBS-LRR resistance gene candidates. Yrr3, which is present in the ABR6 x Bd21 family and effective against all isolates, was mapped to a 400 kilobase region also containing NBS-LRR gene candidates. Agrobacterium-mediated transformation of Yrr1 candidates is underway in Brachypodium for complementation, and in common wheat to test for interspecies transfer of characterized resistance.